The conventional methods for producing air gases in liquid or gaseous form had distinct method architectures. Thus, there could be found:                an air separation apparatus that produced the main constituents (O2, N2, Ar) at atmospheric pressure or slightly higher;        a step of compressing the products using compressors;        an independent nitrogen-liquefaction cycle that allowed all or some of each of the constituents to be produced in liquid form if necessary.        
This configuration allowed a great deal of flexibility of use because each of the three “functions” implemented (separation, compression, liquefaction) could be performed or halted independently without affecting the operation of the other two.
Nonetheless, this configuration suffers from a significant lack of competitiveness, bearing in mind the very high cost of this design which requires one apparatus per function.
The most recent methods for producing air gases, which we term integrated methods, have the advantage that they can combine these three functions into a single equipment. So-called “pumped” apparatuses, including cycles of expanding air or possibly nitrogen, allow one and the same equipment to produce the constituents of air in pressurized gaseous form and in liquid form.
Among these, the methods involving staged vaporization in order to deliver products under pressure, as described in patent EP-A-0504029 or alternatively FR-A-2688052, are particularly attractive because they allow these functions to be combined from a single high-pressure air compressor. The energy efficiency of the whole is comparable with the traditional method and the investment is far lower.
By contrast, the flexibility of production is affected by the “three-in-one” combining of the functions and it becomes more difficult to operate or halt one function without affecting the whole.